1. Field of the Invention
The present invention relates generally to membrane filtration systems and more particularly to seals used in spiral membrane elements of filtration systems.
2. Description of Related Art
Certain types of filtration systems used for removing chemical contaminants and organisms from water comprise one or more filtration elements that are sealed within an enclosure. The enclosure may comprise a canister, a drum and/or a pipe. In particular, filtration systems used for large-scale water treatment can include a series of elements that connect together within a pipe like structure and which direct an inflow of contaminated or impure water through a filter material and onto an outflow pipe or channel. In the example shown in FIG. 1, filtration element 11 in a spiral membrane filtration system comprises a membrane structure that is wound in a spiral. In FIG. 1, permeate carrier sheet 18 is laminated within an envelope of a membrane filtration sheet 19 and adjacent layers are separated by feed spacers 101 and typically enclosed within a hard shell or wrapping to prevent leakage of the inflow and to provide a degree of mechanical stability and strength to filtration element 11. Filtration elements, such as spiral membrane filtration element 11, are typically provided in a substantially cylindrical form and one or more filtration elements 11 can be installed end-on-end within a housing 10 (as shown in FIG. 1C). An inflow fluid 140 is introduced through an inlet under pressure into an end of the system, and enters filtration element 11 at one end 140 and, having passed through membrane 19, exits either as a permeate stream 143, typically through a center pipe or channel 13, or as a concentrate stream 144 which exits from the membrane filtration device. The center pipe 13 is typically coaxial with the enclosure 10 and coupled or otherwise connected with the membrane 19 in a manner that permits collection of the permeate 143. Permeate 143 can be drawn from the system in either direction.
These filtration elements function as membrane filters. Unlike conventional batch mode filtration systems, the described filtration system operates as continuous steady state process. As such the total of all material entering in the feed stream 15 is substantially equal to the summation of all material leaving the filtration device in the two exit streams 143 and 144. Such systems may be used in applications that deliver drinking water, clean or treat wastewater and/or storm water, extract water from sludge, and/or desalinate water such as sea water; in these applications, the dilute permeate stream 143 is the principal product of the system. Conversely the concentrate stream 144 may provide the principal product where the objective is to recover or concentrate a valuable solute.
Spiral membrane elements 11 are used as a means of packaging flat sheet, reverse osmosis membrane 19 in useful separation applications. These elements are typically loaded end to end in a cylindrical housing 10 as shown in FIG. 1C. Process feed flow 140 is introduced at one end of the housing and flows axially 141 through the element 11, with some portion 142 passing through the filter medium 19 to a center collection channel or pipe system 13 from which it is provided as an outflow 143. The concentrated remnant 144 is drawn from a first element 11 into a second element 11 and so on. Concentrate 144 extracted from the system can be processed externally and/or recycled through the system based on system configuration and function. It is necessary to provide a sealing mechanism between successive spiral elements 11 that insures concentrate stream 144 from the first element 11 is passed as a feed stream 140 to the subsequent spiral membrane filtration element 11.
This sealing mechanism can be accomplished using seal plates 12 (shown in more detail in FIG. 1B) that are attached to each end of each spiral element 11. In conventional systems, elastomeric seals are placed in a groove 16 located on an external edge of seal plate 12, in order to prevent escape of fluid into a space between element 11 and housing or vessel 10. Couplings 130 connect successive center channels 13 are typically also sealed using elastomeric seals.
An additional seal 120 may be required between the spiral element 11 and the inner wall of the cylindrical housing 10 to direct the flow 140 into the element 11 itself rather than the annular space between the element 11 and the housing 10. If the flow 140 were not directed primarily into the element structure the velocity of the feed flow over the membrane sheet would be reduced which would impact the separation performance of the membrane sheet. Conventional systems provide an elastomeric seal in a circumferential grooved depression 16 located on the outer surface of a seal plate 12 as shown in sectional FIG. 2A. A commonly used elastomeric seal 24 is shaped in a cup form as shown in FIG. 2B which creates an effective seal but requires that the element be inserted into the housing in one direction as the seal cannot be pushed a reverse direction. A symmetrical elastomeric seal such as an O-ring 26 could be used within the element seal late groove as sown in FIG. 2C. This permits movement in either direction but relies on a greater amount of deformation of elastomeric seal in order to function as an effective seal. This results in greater force needed to insert the element into the cylindrical housing and is the principal reason for the preference for the cupped shaped elastomeric seal.